1. Field of the Invention
The present invention relates generally to ink jet recording apparatuses which selectively emit ink droplets from a recording head onto a recording medium, and is particularly concerned with reducing or eliminating ink flow obstructions caused by air bubbles present in the ink supply path.
2. Description of the Related Art
Typically, an ink jet recording apparatus includes a recording head, an ink container for storing the ink, and one or more supply tubes connecting the ink container to the recording head, thereby forming an ink supply pathway.
The ink container is commonly a plastic case containing a rubber member or porous material. When the ink jet head is used over an extended period of time, the passage of air through the ink container results in a build-up of air bubbles therein, and some air bubbles may eventually flow into the ink supply tubes. Air bubbles can also be introduced to the ink supply pathway supply when the ink jet recording apparatus is subjected to sharp, sudden impacts such as when the ink jet recording apparatus or head itself is accidentally dropped or jarred.
In ink jet recording apparatuses utilizing replaceable ink containers, air bubbles can also be introduced to the ink supply tubes during container replacement. Bubbles can also arise when nitrogen present in the ink vaporizes due to a rise in ink temperature.
When such air bubbles occur, it is preferable to purge the bubbles from the recording head to prevent ink obstruction. This can be accomplished through a recovery device such as disclosed in U.S. Pat. No. 4,967,209 to Hasegawa, et al. As disclosed therein, the recovery device applies pressure to the back of the ink container with a needle-like member to momentarily increase pressure within the ink supply pathway and force expulsion of high viscosity ink near the nozzles or air bubbles within the recording head. This process is commonly referred to as a "priming operation." An alternate conventional recovery method involves covering the recording head nozzles with a cap and applying a vacuum pump connected to the cap to suction the air bubbles and surrounding ink from the recording head, thereby removing the air bubbles with almost equal effect.
With each of these conventional methods, however, there are cases in which the air bubbles inside the ink supply path cannot be removed. Air bubbles may remain in the ink supply path when a stagnation point (a point at which the ink flow is essentially zero even when negative or positive pressure is externally applied) is formed within the ink supply path. Air bubbles gather at this low flow site or stagnation point, and may not be completely removed even by the aforementioned priming process, unless a much greater volume of ink is removed as well.
An example of an ink supply path in which air bubbles tend to collect is described hereinbelow with reference to FIG. 16. FIG. 16 is a partial cross sectional view of a recording head 155 having an intake opening 158. Ink supply tube 153 conveys the ink stored in an ink storage well or tank (not shown) to the recording head, and includes supply opening 152 connecting to the ink receiving portion of recording head 155. After insertion of the ink receiving portion of recording head 155 within supply opening 152, the outside perimeter of the connection is sealed with an adhesive 159, to maintain the connection and form a closed supply path for supplying ink to nozzles (not shown) within the recording head 155.
As shown in FIG. 16, the ink supply path has, at the connection between the recording head 155 and ink supply tube 153, a flow boundary 151 in which the cross sectional area of the ink supply path changes abruptly. Relatively large air bubbles 168 tend to stop at this abrupt flow boundary 151. The contact area between the relatively large air bubble 168 and the side walls 200 of this ink supply path is also large. This large contact area increases the flow drag when the relatively large air bubble 168 attempts to move, and air bubbles capture here may not be sufficiently removed even through conventional priming. To completely remove such bubbles requires transport and emission of a relatively large quantity of ink. This in turn, significantly contributes to undesirable ink waste and collection problems.
Still referring to FIG. 16, when a relatively small air bubble 169 travels within the ink supply path, the smaller air bubble can be absorbed by the relatively large air bubble 168, and the relatively large air bubble 168 may thus grow even larger. In a worst case scenario, the air bubble may grow to cover the entire flow boundary 151 and thus obstruct intake opening 158. When this occurs, ink cannot be supplied to recording head 155, and printing operations cease.
In addition to ink supply paths defining an abrupt change in cross sectional areas as shown in FIG. 16, another type of ink supply path in which air bubbles tend to collect are ink supply paths in which a filter is interposed to prevent the inflow of foreign matter to the recording head. Relatively large air bubbles tend to gather in front of the filter, creating the same problems as described hereinabove.
A known approach for attenuating the occluding effects of these bubbles has been to alter the shape of the ink supply path immediately adjacent to the upstream side of the flow boundary. More specifically, as discussed in Japanese laid open patent application publication JP 05-077440, the ink supply path, which normally exhibits a circular cross section, is hollowed or squared out at the upstream edge of the flow boundary. As a result, this portion of the ink supply path exhibits a rectangular shape having a larger cross sectional area than the remaining circular portion of the ink supply path. Since bubbles collecting at the flow boundary grow spherically, the rectangular cross section of this portion of the ink supply path permits ink to flow in the corner grooves not occluded by a bubble present in the ink supply path.
However, in this structure, there is nothing to prevent the growth of bubbles at the flow boundary. When such a bubble becomes sufficiently large, it can still pinch off the ink flow through the corner grooves through sufficient expansion therein and/or occlusion of the entire orifice or orifices forming the downstream side of the flow boundary.